DS: Gastric Acid Secretion

Question 1

What area the 3 main phases of gastric acid secretion?

Answer 1

1. Cephalic phase
2. Gastric phase
3. Intestinal phase

Question 2

When does the cephalic phase of gastric acid secretion occur?

Answer 2

The cephalic phase of gastric secretion occurs before food enters the stomach due to neurological signals.

Question 3

How much gastric acid is secreted/ produced during the cephalic phase?

Answer 3

Approx. 30% of the total gastric acid secretion

Question 4

What is the main role of the cephalic phase?

Answer 4

Prepares the stomach for its digestive function.

Question 5

What triggers the cephalic phase?

Answer 5

The sight, smell, taste, and thought of food triggers the cephalic phase.

Question 6

How does the cephalic phase occur?

Answer 6

Trigger of action potentials (APs) in the cerebral cortex/hypothalamus which are then relayed to the dorsal motor nuclei (DMN) of the vagus nerve in the medulla and the parasympathetic division of the ANS coordinates a response. Response APs are sent via vagus nerves to the stomach to trigger 4 physiological responses.

Question 7

What are the 4 physiological responses triggered by the body to increase gastric acid secretion by the stomach?

Answer 7

1. Ach secreted by vagal post-ganglionic motor neurons bind to M3 receptors and stimulate hydrogen ion secretion into the lumen of the gastric glands.
2. Ach released from the axon of collaterals of the vagal cholinergic post-ganglionic neurons diffuse through the lamina propria to adjacent ECL cells; release histamine which targets H2 receptors on parietal cells to increase hydrogen ion secretion.
3. P eptidergic post-ganglionic vagal efferent motor neurons release Gastrin Releasing Peptide (GRP); release gastrin which either 1. binds to CCK2R receptors or 2. Triggers histamine release.
4. Vagus nerve inhibits the release of somatostatin from D cells to remove background inhibition of histamine and gastrin release.

Question 8

Describe the process of negative feedback for gastric acid secretion

Answer 8

Low luminal pH stimulates antra D cells to secrete somatostatin which inhibits Gastrin secretion from G cells

Question 9

How long does the gastric phase last for?

Answer 9

3-4 hours

Question 10

What triggers the gastric phase?

Answer 10

Local neural and hormonal mechanism, triggered by the entry of food into the stomach.

Question 11

How much gastric acid is produced during the gastric phase?

Answer 11

Approx. 50-60% of total gastric acid secretion

Question 12

What is the role of the gastric phase?

Answer 12

To maintain gastric acid secretion in response to food entering the stomach

Question 13

How is the gastric phase triggered? What happens during the gastric phase?

Answer 13

Food entering the stomach causes distention which activates the mechanoreceptors in the gastric mucosa that activate a vagal afferent pathway from the stomach to the DMN. The DMN integrates the sensory inputs and transmits APS through the vagal efferent pathway to the stomach.

Distension of the gastric mucosa also activates a local ENS pathway (short myenteric reflex) that releases ACh to stimulate further acid secretion by parietal cells.

Question 14

What food products trigger the production of gastrin?

Answer 14

Partially digested proteins (peptones), caffeine, amino acids (phenylalanine and tryptophan) and rising pH stimulate the release of gastrin from enteroendocrine G cells

Question 15

What is the action of gastrin of smooth muscle cells?

Answer 15

The release of gastrin activates vigorous smooth muscle contractions

Question 16

What is the role of the intestinal phase?

Answer 16

Responsible for the down regulation in gastric acid secretion to protect the stomach and the small intestine.

Question 17

How much gastric acid is produced during the intestinal phase?

Answer 17

Approx 5-10% of total gastric acid secretion

Question 18

What are the 2 components of the intestinal phase?

Answer 18

1. Excitatory phase

2. Inhibitory phase

Question 19

What happens during the excitatory phase of the intestinal phase?

Answer 19

Partially digested food in the duodenum stimulate intestinal mucosal cells to release Gastrin-like hormone (enteric Gastrin) which briefly stimulates HCL secretion from parietal cells.

Question 20

What happens during the inhibitory phase of the intestinal phase?

Answer 20

Enterogastric reflex is initiated as the intestine distends with chyme containing fats, etc. This inhibits the DMN in the medulla and inhibits local enteric reflexes. Activates sympathetic nerves to cause the pyloric sphincter to contract to prevent further chyme entering the SI.

Question 21

What are the role of fats in down regulating gastric acid secretion?

Answer 21

Fats stimulate enterogastrones, secretin, CCK, VIP and GIP release from the intestinal mucosa, and they all help to inhibit gastric acid secretion to protect the small intestine.